Generally, a bicycle derailleur, as disclosed in Japanese Utility Model Publication Gazette No. Sho 55-10217, is provided with a base member, a linkage mechanism comprising a support member, a pair of linkage members and a movable member, and a chain guide for guiding a driving chain from one sprocket to another of a multistage sprocket assembly mounted to the bicycle. The support member is supported swingably to the base member, and the chain guide is swingably mounted to the movable member through a horizontal shaft. A spring is interposed between the movable member and the chain guide, thereby applying tension to the chain guide.
The base member is fixed to a fork end of the bicycle frame, with the chain guide being disposed radially outwardly of the multistage sprocket assembly.
The conventional derailleur which includes a chain guide capable of being adjusted in its axial position with respect to the multistage sprocket assembly is well-known as disclosed in Japanese Patent Publication Gazette No. Sho 56-99869.
The conventional derailleur is constructed such that the movable member comprises a pair of connectors for connecting the linkage members and a pair of mounting members each having a non-round bore. A rotor having a threaded bore is supported rotatably only between the mounting members, the horizontal shaft is mounted rotatably to the horizontal shaft. A tension spring is provided around the outer periphery of the horizontal shaft, and a screw-thread provided at the outer periphery of one end of the horizontal shaft is supported axially movably only to non-round bores in the mounting members. The rotor and a lock nut screw with the screw thread at the horizontal shaft. As a result, the rotor is rotated to axially move the horizontal shaft together with the chain guide, thereby enabling the chain guide to be adjusted in its axial position with respect to the multistage sprocket assembly.
In the conventional derailleur, the rotor screws with the screw thread at the horizontal shaft and the tension spring is supported to the horizontal shaft at the reverse side to the screw thread, thereby increasing the overall length of the derailleur, and in turn the derailleur has increased size at the horizontal shaft position, thereby creating a problem in that the derailleur is prone to hit external objects. The horizontal shaft, which is supported to the mounting members axially movably with respect to the movable member, and the movable member itself, are easily broken by striking such external objects. Also, the lock nut is screwed with the end of the horizontal shaft to prevent a backlash thereof, thereby increasing an axial length of the horizontal shaft, and in turn the size of the derailleur is further enlarged. Furthermore, the horizontal shaft and tension spring are exposed to the exterior, thereby being non-protectable.